The first package:

Note: Grade three students can draw line segments to help them understand. The dotted line indicates used, and the solid line indicates remaining. By comparing these two packages, we can see that the part used in the first package is 6- 1=5 less than that in the second package. Where 1 time is 1, and 6 times is 6 copies. )

So the formula is

Length of a serving: (396-156) ÷ (6-1) = 48 (decimeter)

Full length: 48+396=444 (decimeter)